Ceramic matrices or sets of metallic meshes are used as radiating elements in radiant gas heaters. A radiant gas burner with a radiating element in the form of a two-layer ceramic matrix is described in U.S. Pat. No. 4,889,481. A downstream air-gas mixture motion is therein described, wherein a burner body comprises a first layer of porous ceramic material adjacent an inlet side and a second layer of porous ceramic material adjacent an outlet side. The first layer is 0.25 mm thick and possesses a porous structure with pore diameter of between 0.01 and 2.5 mm. The thickness of the second layer is 1.25 cm and also includes a porous structure with a pore diameter of between 1.25 and 10 mm. The shortcomings of the above described burner include high flow resistance and brittleness of the material layers (ceramic matrix).
Industrial IR burners with low carbonic oxide (CO) and nitric oxide (NO) content in combustion products are known, for example, from Russian patent publication no. 2,084,762. The typical industrial IR burner, described therein, consists of a casing, IR deflector, an injector with a nozzle and mixer, a reflector with a shelf, a radiating ceramic mouthpiece, and a mesh. Accommodating the reflector at some distance from the injector's outlet allows uniform combustion along the whole burner's surface and reduces the carbonic oxide and nitric oxide content in the combustion products. One of the shortcomings of this type of burner is that the ceramic mouthpiece poorly withstands thermal and physical shocks and thus, is of little use in burners for domestic gas cookers. Additionally, burners with ceramic mouthpieces have a limited power control range.
An industrial radiant burner is also known (see for example U.S. Pat. No. 4,437,833) to work in heat units using natural and liquefied gas of medium pressure. This type of industrial radiant burner usually consists of a casing, a nozzle unit, an injector, a dissector, an emitting orifice, and a screen mesh. The emitting orifice can be a unit of 32 holed ceramic plates, for example, having a fire channel diameter of 0.85 mm. The casing can include an emitter consisting of refractory mesh and a reflecting screen of metallic wire fixed in the casing. Combustion occurs in between the refractory mesh and the reflecting screen. To provide uniform air-gas mixture flow distribution, dissector plates are accommodated inside the casing. The major drawbacks of the industrial radiating burner, as of all burners with ceramic radiating elements, are insufficient resistance to physical and thermal shocks, small power control range, and high flow resistance.
Using metallic meshes, instead of ceramic radiating elements, has found application in radiating burners for hot-water boilers. These types of burners consist of a flat holder with a supply gas line, see for example U.S. Pat. No. 5,474,443. There is a radiating element fixed on the holder that constitutes a metallic mesh of hemispherical shape and at least a one holed gas-distributing surface of the same shape. Air-gas mixture combustion occurs above the mesh surface. To obtain sufficiently complete combustion above the surface of the metallic mesh, one needs an object returning part of the mesh emission back towards the metallic mesh. Such an object in the burner considered can be a boiler furnace surface surrounding the burner, thereby limiting this type of application in other devices.
Other challenges associated with metallic emitters include a larger portion of radiant energy from the emitter going in the opposite direction of the heat receiver, thereby resulting in undesirable heating of the burner's casing.
Another metallic mesh IR burner is known and comprises a set of metallic meshes located downstream from the flow of the air-gas mixture. A first distributing mesh converts a dynamic component of the pressure into a static one. At the same time, the first mesh shields the burner's casing against backspattered emission. A second and a third mesh are coupled as one pack and form a burner's emitter. A fourth protective mesh guards the emitter against mechanical damages. The burner also accommodates a gas nozzle and an injection air-gas mixer located in parallel with, and under, a distributing mesh (O. N. Bryukhanov et. al., Unified Metallic-Mesh IR Burner, Gazovaya Promyshlennost, N 3, 1985). In the given burner, the first mesh is made of a punched metallic plate. Efficiency of “trapping” backscattered emission via the mesh(es) is directly related to the total area of the holes made in the plate, i.e. with a real plate cross-section. Increasing the efficiency of backscattered emission “trapping” requires lower “real” plate cross-section. Meeting this requirement will lead to high flow resistance of the gas dissector and consequently to lower working capacity of the burner in general. This is a significant drawback of the given design of the gas dissector.
Another disadvantage of the aforementioned burner is that the location of the air-gas mixer does not ensure uniform distribution of the air-gas mixture on the emitter surface and causes additional flow resistance. Further shortcomings of the burner also include the fact that structurally reliable automatic ignition can't be provided. Ignition can be done from outside the burner only, i.e. from above the latter downstream air-gas mixture flow towards the mesh. If one uses spark, resistive or another ignition, during the burner operation, the ignition unit will be within an undesirable high temperature zone.